N-Glycan biosynthesis

The term ‘Glycan’ refers to oligosaccharides or polysaccharides. They also refer to glycoconjugates such as glycolipids and glycoproteins. Two different types of glycans attached to proteins are N-glycans and O-glycans. N-glycans, more precisely N-linked glycans are attached to the Asp residue of the tripeptide sequence of Asp-X-Ser/Thr (X is any amino acid except Pro) of proteins. In most eukaryotes, the N-glycan anchors are derived from a 14-sugar oligosaccharide containing 9 mannose, 3 glucose and 2 N-acetylglucosamine molecules. This process begins with the addition of 2 N-acetylglucosamine to dolichyl-phosphate in the cytoplasmic side of endoplasmic reticulum membrane. After the addition of 2 N-acetylglucosamine and 5 mannose residues, the remaining process continues in the internal side of ER. Then the enzyme oligosaccharide-protein glycosyltransferase links this glycan anchor to the above mentioned Asp residue of polypeptides in the lumen of ER. The glycan anchor can then be processed (by trimming and extension of sugar residues) to produce various types of N-glycans. This process takes place in golgi apparatus. The main processing includes removal of glucose and mannose residues and addition of N-acetylglucosamine, N-acetylneuraminic acid and fucose residues. The attachment of the glycan anchors is important for the proper folding of many proteins.

 

This pathway was not reconstructed for Plasmodium falciparum in MPMP. This pathway is reconstructed with KEGG reference pathway of N-glycan biosynthesis as template for Coccidians Toxoplasma gondii and Neospora caninum. The analysis of T. gondii and N. caninum gene models showed that it has all the enzymes necessary for the formation of 14-sugar core anchor from dolichyl-phosphate. The enzyme which catalyses N-linked glycosylation (oligosaccharide-protein glycosyltransferase) is also present. In addition they also possess the enzymes for the trimming of 3 glucose residues and this forms high-mannose oligosaccharide N-glycans. Except MAN1, all the enzymes required for the removal of 4 mannose residues and addition one more N-acetylglucosamine sugar (forms complex oligosaccharide N-glycans) is also present in the gene models. The two enzymes which are present in P. falciparum (dolichol metabolism pathway) and involved in conversion of dolichol phosphate from dolichol (dolichol kinase) and dolichol diphosphate (dolichyldiphosphatase) were absent in the release 7 gene models of T. gondii and N. caninum. Of these, dolichol kinase was recently identifiedin release 8 gene models of T. gondii and it also has an ortholog in N. caninum.

 

Enzyme EC Number Gene id
alpha-1,3-mannosyl-glycoprotein 2-beta-N-acetylglucosaminyl transferase (MGAT1) 2.4.1.101 NCLIV_039610
Dolichyl-phosphate beta-glucosyltransferase (ALG5) 2.4.1.117 NCLIV_059710
Dolichyl-diphosphooligosaccharide-protein glycotransferase (STT) 2.4.1.119 NCLIV_021120
Dolichyl-diphosphooligosaccharide-protein glycotransferase (STT) 2.4.1.119 NCLIV_031690
Mannosyltransferase (ALG3/9/12) 2.4.1.130 (Entry transferred to 2.4.1.258, 2.4.1.259, 2.4.1.260 & 2.4.1.261) NCLIV_004260
alpha-1,2-mannosyltransferase (ALG11) 2.4.1.131 NCLIV_063650
Mannosyltransferase (ALG2) 2.4.1.132; 2.4.1.257 NCLIV_045600
N-acetylglucosaminyldiphospho dolichol N-acetylglucosaminyltransferase (ALG14) 2.4.1.141 NCLIV_037860
Chitobiosyldiphosphodolichol beta-mannosyltransferase (ALG1) 2.4.1.142 NCLIV_031100
Dol-P-Glc:Glc2-Man9-GlcNAc2-PP -Dol alpha-1,2-glucosyltransferase (ALG10) 2.4.1.256 NCLIV_024260
Dol-P-Glc:Glc1-Man9-GlcNAc2-PP -Dol alpha-1,3-glucosyltransferase (ALG8) 2.4.1.265 NCLIV_057630
Dol-P-Glc:Man9-GlcNAc2-PP-Dol alpha-1,3-glucosyltransferase (ALG6) 2.4.1.267 NCLIV_025590
Dolichyl-phosphate beta-D-mannosyltransferase (DPM1) 2.4.1.83 NCLIV_068040
Farnesyltransferase 2.5.1.29 NCLIV_038220
Dolichol kinase 2.7.1.108 NCLIV_035680
UDP-N-acetylglucosamine-dolichyl-phosphate N-acetylglucosaminephospho transferase (ALG7) 2.7.8.15 NCLIV_019070
Mannosyl-oligosaccharide glucosidase (GCS1) 3.2.1.106 NCLIV_017340
Mannosyl-oligosaccharide 1,2-alpha-mannosidase (MAN1) 3.2.1.113 Missing in annotation
Glucan 1,3-alpha-glucosidase (GANAB) 3.2.1.84 NCLIV_007580
Dolichyldiphosphatase 3.6.1.43 Missing in annotation

 

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Sources and fates of metabolites

 

Substrate Source pathways Product Fate pathways
Dolichol Host    
Farnesyl-PP Terpenoid metabolism    
Isopentenyl-PP Isoprenoids metabolism    
UDP-Glucose Starch and galactose metabolism Glucose Glycolysis, Starch and galactose metabolism
UDP-N-acetyl-glucosamine Aminosugars metabolism    
GDP-D-mannose Mannose and fructose metabolism